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Because the lepton number conservation law in fact is violated by chiral anomalies, there are problems applying this symmetry universally over all energy scales. However, the quantum number B − L is commonly conserved in Grand Unified Theory models.
Exact conservation laws include conservation of mass-energy, conservation of linear momentum, conservation of angular momentum, and conservation of electric charge. There are also many approximate conservation laws, which apply to such quantities as mass , parity , [ 1 ] lepton number , baryon number , strangeness , hypercharge , etc.
A much stronger conservation law is the conservation of the total number of leptons (L with no subscript), ... Lepton flavor number Mass [MeV/c 2]
In some theories, such as the grand unified theory, the individual baryon and lepton number conservation can be violated, if the difference between them (B − L) is conserved (see Chiral anomaly). Strong interactions conserve all flavours, but all flavour quantum numbers are violated (changed, non-conserved) by electroweak interactions .
Conservation of angular momentum. Conservation of total (i.e. net) lepton number, which is the number of leptons (such as the electron) minus the number of antileptons (such as the positron); this can be described as a conservation of (net) matter law.
Conservation of energy and momentum are the principal constraints on the process. [1] All other conserved quantum numbers ( angular momentum , electric charge , lepton number ) of the produced particles must sum to zero – thus the created particles shall have opposite values of each other.
Three antiquarks of different anticolors, giving an antibaryon with baryon number −1. The baryon number was defined long before the quark model was established, so rather than changing the definitions, particle physicists simply gave quarks one third the baryon number. Nowadays it might be more accurate to speak of the conservation of quark ...
The anomalies that would break baryon number conservation and lepton number conservation individually cancel in such a way that B – L is always conserved. One hypothetical example is proton decay where a proton (B = 1, L = 0) would decay into a pion (B = 0, L = 0) and positron (B = 0, L = –1). The weak hypercharge Y W is related to B – L via